Lock camera on an axis?
To align the viewport to a specific axis you can use the Numpad:


Y axis - Numpad 1
X axis - Numpad 3
Z axis - Numpad 7


You can use the Ctrl key in conjunction with these keys to invert the direction you are looking along the axis. To move along a certain axis while preserving your viewport orientation:


To move along the Z axis - hold Shift while scrolling.
To move along the X or Y axis - hold Ctrl while scrolling.
To zoom along the X or Y axis - scroll.